Coated aluminum



PatcntedJuly26,l932

UNITED STATES PATENT OFFICE BENGSTON, OF INDIANAPOLIS, INDIANA,ASSIGNOR, BY NIESNE ASSIGNMENTS, 1'0 ALUII'JIUI COLORS INCORPORATED, 0FINDIANAPOLIS, INDIANA, 'A CORPO- BLTION OF INDIANA COATED m Drawing.

This invention relates to coated aluminum; and it comprises an articleof aluminum or of aluminum alloy wholly or partially covered with anintegrally lmited coating containing ALL-O S0 and ILO in proportionscorresponding to a sub-sulfate and produced by making said article ananode in sulfuric acid of a specific gravity between 1.5 and 1.? with alow voltage current; said coating being of substantial thickness,usually of the order of a thousandth of an inch (0.0001 to 0.0015).being minutely porous or pervious, and taking up dyes with readiness andwith substantial uniformity at all points, and evincing no crystallinityunder X-ray examination but having a laminae structure with an extremelyhard and extremely thin, glazelayer next the metal and a much thickerbut less hard outer layer integrally united thereto. the hardness of thelayer as a whole being great enough to permit boiling and polishing andit also comprises such an article, dyed or undyed, having the poresfilled to give a composite impervious coating; all as more fullyhereinafter set forth and as claimed.

As is well known, metallic aluminum is always covered with a film ofaluminum oxid formed by the action of air. This film is impervious andis self-renewing in the sense that where it is removed, cracked orbroken it is, practically at once, renewed by the action of the air.Being formed at the expense of the metal and being wholly impervious itis of necessity extremely thin, being possibly mono-molecular.Naturally, where it forms further attack on the underlying metal isprecluded. While this film is an excellent protection against corrosionand is not unsightly, its existence practically renders impossible theordinary decorative treatments which can be applied to other metals. Itis, for example, diflicultto make different finishes by different waysof boiling or polishing; and paints, lacquers and varnishes do not holdwell to it; both because its impervious nature prevents anchorage andbecause mostcoating materials do notcohere to alumina; do not wet it, soto speak. For reason aluminum articles are seldom decorated in any way.Plating Appficafionfiled Inc 11, 1930. Serial No. 460,543.

aluminum with other andmore ornamental metals is quite dificult.

In the prior art, attempts have been made to improve on the natural oxidfilms; artificial oxid films being formed in various Ways, mostlyelectrolytic. The object being to protect the metal, impermeablecoatings are sought and these are necessarily extremely thin. Withformation of an impervious coating, attack on the underlying metalceases. These films are usually quite hard and rigid; and are too thinto be bufied, polished or mechanically finished. They do not afford agood base for decorative finishes. While staining and dyeing have beenproposed, neither has been successful.

Aluminum articles under the present invention carry a hard, perviouscoating formed under oxidizing conditions at the expense of theunderlying metal; this coating being of substantial thickness, say0.0001 to 0.0015 inch, and being composed of A1 03, S0 and H 0 in someform of combination. lVith aluminum alloys containing silicon, thecoating may also contain a little SiO While the coatings are pervious,the pores are not visible, being of sub-microscopic magnitude. Thecoating has a more or less developed structure, there being arelativelythick hard outer layer and a relatively thin glaze-like still harderunderlayer next the metal. The glaze layer is sometimes only 5 per centor so of the total thickness of the coating. The coating as a Whole ismechanically strong and can readily be bullied or polished to a smoothfinish. In color the coating is White or light gray, coatings high insilica being sometimes a darker gray. The coating withstands heat welland-I sometimes bake it during or prior to finishing, It takes dyesreadily.

In finished articles under the present in vention, I may simply butt andpolish the described coating. In buifing and polishing, the stearic acidor grease of the ordinary bufiing compositions suffices to fill thepores and make the coating impervious. .However,

I sometimes use special impregnants such as lanoline; Usually, but notalways, I- dye or stain the coating.

In another and copending application, Serial No. 463,351, I havedescribed and claimed a method of producing relatively thick coatings ofthis type upon aluminum and aluminum alloy articles. In this method,aluminum is made an anode in a somewhat diluted sulfuric acid, the acidusually being around per cent of H 80... The result is formation of acoating having the described characteristics. In this method ofoperation the first result of the passage of current is the productionof an extremely thin coating of flinty hardness. This film, however, ispervious and attack proceeds with building up of an overlying layer; themain body of the coating of the present invention. There is at first asurge of current and then the current settles down to a steady value.Within limits, any thickness of coating desired may be produced at theexpense of the underlying metal which is correspondingly etched. Thecoating is porous and the pores extend to the actual metal. The coatingis at first transparent but becomes translucent and whitish as itthickens. The coating is, as stated, susceptible of butting andpolishing by the usual tools; being polished with readiness. Itwithstands bending, stamping and punching to a reasonable extent.Although actually pervious' the coating affords a reasonable degree ofprotection to the underlying metal because of the extreme fineness ofthe pores which are, as stated, of sub-microscopic disuflices to sealoff these pores. Lanolin, petrolatum, beeswax, etc. may be used. It iswhich may be due to some sort of adsorptive action by the aluminumcompounds formingthe coating. It is also noted that the stearic acid andgrease of buffing compositions are similarly takenup. I

i The coating treatment may be applied to flat aluminum metalsubsequently stamped into form for certain types of articles and.

specific uses or it may be applied to the finished articles. The shapeof the article makes no particular difierence in coating; deep recessesbeing coated as readily as flat surfaces. Before coating the articleshould be finished v and thoroughly cleaned.

The coatlng contains aluminum oxid, S0

and water of hydration. The composition of thecoating as a whole oftenempirically corresponds to about 3Al O .5H O.SO that is to a hydratedsubsulfate, or extremely basic sulfate, of aluminum. No evidence ofcrystalline structure can be found by X-ray examination. The coating asa whole may contain ilfwalds of 16 per cent S0 and 18 per cent ,O. Thethickness of the coating can readimensions. A little grease or oil ofany kind 7 ly be made around 0.001 inch; or about five times thethickness of the usual nickel plating. On careful examination thecoating is found to be, so to speak, graduated in hardness from-surfaceto base. Mechanically, the aluminum, the glassy linking layer, and theoutside layer behave as if they are integrally united. The describedcoating may be produced by the method of the acknowledged application,or byother methods.

The coating is colorless or grayish white and can be readily andpermanently dyed, taking up and holding those dyes which are customarilyused with mordants. In other words, the coating is readily convertedinto a lake of any desired color or hue. Other mit bufling and polishingto an attractive finish. Buffing,; polishing and mechanical finishing ofthe coated article is done after dyeing. It is found that the coating,both before and after bufiing, will take up oils and oily materials.Colored oils give various decorative effects. Paints, stains, varnishesand lacquers applied to the article give a good bond to the coating.Various-chemical treatments are possible. The coated article may. forexample, be immersed in a fairly concentrated solution of sodiumdichromate and allowed to drain and immersed in a solution of leadacetate. ganic pigment, is chemically precipitated within. the pores ofthe coating by the resulting metathetical reaction. By treating withresin soaps, resinates may be produced in the pores. This isadvantageous where an additional keying action is desired in lacqueringor enameling.

The strength of the sulfuric acid electrolyte used in the anodictreatment is of considerable importance. The best quality of coatingsunder the present invention are now being obtained in 64 to 65 per centacid. A range of acid strength between 60 and 77 per cent H 80 that is,a specific gravity of 1.5 to 1.7 gives excellent coatings of the typehere described and claimed. l/Vith acid of higher specific gravity than1.7 .the conductivity is so small that it is difficult to secureefliclent current densities, while lower concentrations of acid, between35 and 60 per cent, cause excessive etching and attack on the aluminumwith poor coatingsas a'. result. In making coatings with concentrationsof sulfuric acid between 60 and 77 per cent, it has been found highlyadvantageous to add some glycerin to the electrolyte. This aids inreventing local action and makes for unifbrmity of coating. Presence ofglycerin is useful in retarding attack of the acid on the coating andsolution of metal. I use ordinarily about one part by volume of glycerinto 15 parts Lead chromate, an inorb volume of 1.5 to 1.7 acid. Variousother dition agents, such as cresol and other phenols, may be used.Phenols when used are advantageously first sulfonated.

Other acids, such as nitric acid, are not here the equivalent ofsulfuric acid since they cause surface toughening and attack withoutproduction of the type and composition of coating here desired;

In operation, the apparatus in which the anodic coating is effected ismost convenlently made of stoneware or other ceramic or vitreousmaterial. However, containers of wood or metal, lined with materialsresistant to sulfuric acid, such as pitch, rubber or sheet lead, may beused. The cathodes may be of any convenient material, but I findcathodes of ordinary chemical lead best adapted. The size and shape ofthe cathodes, of course, vary according to the work to be done; but formany purposes they may be located on the side walls of the bath andcover about 50 per cent of; the side wall area. With lead linedcontainers the lead linlng may very conveniently serve as a cathode. Thearticles to be treated may be hung in the bath upon the usual conductingrods. Pointed clamps may be used with suflicientpassage of current sothat the uncoated area in the finished article is merely animperceptible spot. The direct current voltage used is ordinarily about12 volts between terminals. With a voltage of this order no rheostats orregulating devices are necessary. The bath 1s cooled in any. of theordinary ways. With large baths, cooling coils or cooling jackets aregenerally desirable. Circulation of the electrolyte, with agitationthereof, in well known ways, is also desirable. Heat is evolved in theoperations. It is best to keep the temperature between 20 to 30,preferably at 25 C. The anode current density, with the usual spacingbetween anodes and cathodes, is ordinarily between 6 and 10 ameres persquare foot of surface of the article ing treated. a

While I have described making the aluminum an anode in a sulfuric acidbath with employment of direct current of the usual type used inplating, an alternating current may be used in lieu of direct current.The rectifier action of aluminum makes this feasible. I have securedgood coatings with low voltage alternating current. Even 8 volts havesufiiced. In some instances this'applicability of alternating current isadvantageous since a small step-down transformer can be used withordinary 60 cycle lighting current in lieu of the more complex equipment necessary in securing low voltage direct current. In usingalternating current, aluminum articles to be coated may form both ingtreatment. When a lustrous coating is wanted the aluminum should bebuffed before'cleaning. In any event, grease and other forei mattersshould be removed. The aluminum articles can be given a coatin ofornamental design by blocking ofi' port1ons of the surface withasphaltum base stopofl or other acid resisting materials and protectingdesigned portions from the anodic treatment.

For most purposes the time usually required for the anodic treatment toproduce the desired coating as above described is 10 to 30 minutes; thetime varying with diiferent alloys in producing a given thickness ofcoating. However, a useful coating may usually be formed in 10 minutesor less, and an extended time even of several hours does no harm.

Agitation during the coating operation maintains uniform temperatureconditions and gives better coatings. It is also useful in avoiding theformation of gas pockets in curved or recessed parts of the article.

If the work is not completely clean there is apt to be unevenness andvarying thicknesses of surface coating with consequent uneven colorationby subsequent dyeing. Defective contacts with individual pieces inprocessing a batch of articles may require their reprocessing. To removethe coating for reprocessing the article a 5 to 10 per cent hydrofluoricacid dip is most satisfactory. On immersion the coating rapidlydissolves, solution taking perhaps 15 seconds. There is then a vigorousevolution of gas as the acid reaches the metal. When this occurs thearticles are removed and washed. Because of the stated etching it isfrequently necessary to rebuff or repolish the article. When cleaned andWashed, and if necessary repol-- ished, the article is ready forreprocessing.

The anodic treatment described usually gives a coating of about 0.0005inch thick. With a somewhat greater time of anodic treatment thethickness of coating may be 0.0015 inch or more. These thicknesses aresufficient for all ordinary purposes, and while somewhat thickercoatings may be secured the result is more attack on the underlyingaluminum.

In coating, some of the metal goes into solution in the acid as aluminumsulfate and after a time the acid becomes saturated and depositscrystals. To dispose of this crystallized sulfate and keep down thetemperature of the bath, it is usually expedient to keep a portion ofthe bath in cyclic circulation to and through a filter and cooling meansand back to the bath. A leadlined pump may be used.

the anodic treatment are as follows: It is a non-conductor ofelectricity when dry and Some propertiesof a coating produced by ioiled; it is readily reactive with, absorbs and permanently holdsorganic dyes; it is resistant to wear; it stands bufling and takes a hi"h polish; it withstands heating without lifting or breaking away fromthe metal; it binds paints, oils, enamels, lacquers and the like; itshows no definite crystalline structure under X-ray examination; and itis a coating of reasonable thickness rather than a film.

-The coating is apparently entirely non-crysrepresented about 10 percent of the total film.- The coating is resistant to heat, withstandingtemperatures up to, say, 300 (3., without shelling or cracking andwithout much loss of water. With well rinsed material, the amount ofmicroscopic moisture given atl00 C., is very little. The rest of v the H0 is more permanently held. The

- to neutralize retained electrolyte or free acid.

Coatings which are simply dried or baked,

coating is tolerably resistant to acids but loses sulfuric acid inalkaline liquids. It gives up sulfate to boiling Water but two hoursboiling with distilled water are required for completion of the action.Rinsed, dry material does not corrode in the air and with the poresplugged or filled, corrosion by sea Water does not occur. Asillustrating the adsorptive power of the coating material, it may bementioned that on application. of lacquers containing dyes, some of thedye sometimes migrates into the underlying coating.

When taken from the coating bath, the coating is well rinsed to free itof acid. It is then, usually, dried. It may then be baked. Baking makesit somewhat more porous and less adapted for taking up dye. Prior to.drying and baking,various' dips may be used but not otherwise treated,are useful in some relations, as on aluminum pistons.

lVhere the coating is to be dyed, it is in general used wet, i. e.,after rinsing and, in some cases, after a neutralizing dip. The rinsed,wet coating dyes readily but displays its own specific peculiarities indyeing. In

a general way, all dyes which can be used with an alumina mordant can behere used. Acid dye baths are not, in general, applicable; even thosewhich are acid with acetic acid. In neutralizing dye baths,neutralization with sodium carbonate is better than neutralization withammonia. The presence of ordinary salines employed in dye baths asassistants, such as sodium sulfate, is not injurious. An addition ofsodium chromate is often useful. In. articles exposed to light,

lightfastness of the dye is highly important; and for automobilefinishes and the like, even with dyes ordinarily regarded as lightfast,the use of protective lacquers or varnishes, shutting out ultra-violetlight is sometimes necessary. However, dyes which are really lightfastcan be used. Dyeing is done in the usual way with'the usual baths andrequires usually 30 minutes to an hour. Exhaustion of the bath isgenerally good. Articles to be dyed are suspended in the dye bath fromaluminum rods or on aluminum racks or with aluminum wire. After dyeing,the article is rinsed and airdried. In some cases, it is bufi'ed afterdrying, using a'standard white lime composition of the type used fornickel bufiing. In so doing, the binder of the bufling composition,which is usually stearic acid or tallow, 1s taken up, more or less, inthe pores. Sometimes, the dyed coating, after air-drying or oven drying,is further treated with fillers which may be liquid, waxy or solid. Forinstance, waxes of varying melting points or solutions of waxes invarious organic solvents or slushes which can carry greases or resinsinto the body of the film, may be used. After such procedure, bufiing isnot usually resorted to, but the article is merely dried with clean ragsand then polished by hand.

In the selection of a filler, the choice depends on the kind ofresistance required of the treated article. A finished article with acoat that is required to contain foodstufis,

such as refrigerator trays, may be treated with a casein filler. A widevariety of chemical dips can be employed in lieu of casein. It is usualnot to bufiI' after a filler has been employed, as the work drags underthe buflin wheel and sometimes is smeared, instead 0 being even incolor. Where it is necessary to get a good finish after dyeing, bufiingis resorted to without the colored piece being previously treated. Thearticles, after buffing, can be made impervious with certain oils andsolutions. I have used molten lanolin and lanolin dissolved in benzol. Ihave also used petrolatum and paraflin. Beeswax and similar materialsare very good.

Various kinds of decorative or ornamental effects are possible with theanodic sulfuric coating. Parts of an article may be coated, dyed, oiled,painted, varnished or lacquered, while other parts are left with theirnatural surfaces, or these other parts may be differently dyed, oiled,painted, varnished or lacquered.

Generally I cover the whole article with a coating; but in special casesI localize the coating, thereby producing special articles. For example,in aluminum cooking utensils it is desirable that the sides be polishedmetal to lessen'radiation and cooling while it is desirable that thebase be black or dark colored to facilitate heating on the stove. Withaluminum cooking utensils therefore I customarily stop off all thesurface with celluloid, asphaltum base, paraflin or suitable stop- 7ping olf material, except the bottom or the heat receiving surface. ThisI coat as previously described, dye with a black or dark dye and thenfill with wax or the like. In heating the kettle or the like this waxcustomarily goes above its melting point but 10 it does not ooze out orleave the article if the coating is properly made.

The present invention is applicable to any article of aluminum or'itsalloys. In the case of duralumin and other alloys containing silicon,the coating carries some silica, but its properties are about the same.

In addition to the cooking utensils mentioned, the present inventionfinds utility in application to automobile and aeroplane parts andaccessories. Beautiful and permanent finishes can be given to sheetmetal without the use of varnishes or lacquers. The sheet metal may begiven a pebble or leather finish, then coated and finally dyed andfilled. Ba-

diators may be coated inside and outside; the inside coating being leftundyed while the outside is dyed in a dark color to promote radiation. 7

Aside from its advantageous mechanical properties, the present coatingowes most of its value to its highly developed adsorptive power; a powerwhich enables it to take up and hold oils, coloring matters, etc.against withdrawal. I attribute this high adsorptive power in part tothe large amount of fixedly contained SO or basic aluminum sulfate asthe case may be. This basic sulfate is in a formresisting action by coldwater and it is extremely active as an adsorbent.

In the article of the present invention, the

fixedly held S0 may range between, say, 16 per cent and 23-24 per centof the coating.

What I claim is 1. An article ofaluminum or alloy thereof having formedthereon an integrally united coating containing A1 0 S0 and H 0, havinga thickness of the order of a'thousandth of an inch obtained by anodicoxidation of said article in a bath of sulfuric acid between 1.5 and 1.7specific gravity and being of glassy hardness next the metal withlessened hardness toward the surface, said coating-being resistant towater and to wear,

minutely porous, adsorptive, capable of being buffed and polished andcapable of being dyed with level a pearance.

2. An article 0 aluminum or alloy thereof having formed thereon anintegrally united coating containing A1 0,, SO; and H O,

' having a thickness of the order of a thousandth of an inch obtained byanodic oxidation of said article in a bath of sulfuric acid between 1.5and 1.7 specific gravity and being ing being resistant to water and towear, minutely porous, adsorptive, and having its pores filled withimpregnating matter.

3. An article of aluminum or alloy thereof having formed thereon anintegrally united coating containing A1 0 S0 and H 0, having a thicknessof the order of a thousandth of an inch obtained by anodic oxida tion ofsaid article in a bath of sulfuric acid between 1.5 and 1.7 specificgravity and being of glassy hardness next the metal with lessenedhardness toward the surface, said coating being resistant to wear,minutely porous and adsorptive, being buffed and having its pores filledwith oily impregnants.

4. An article of aluminum or alloy thereof having formed thereon anintegrally united coating containing A1 0 S0 and H 0, obtained by anodicoxidation of said article in a bath of sulfuric acid between 1.5 and 1.7specific gravity, said coating having a thickness of the order of athousandth of an inch and being of glassy hardness nextthe metal withlessened hardness toward the surface, said coating being resistant towear. minutely porous, adsorptlve and being dyed and buffed.

5. An article of aluminum or alloy thereof having formed thereon anintegrall united adsorptive coating containing A1 8 and H 0 and anamount of SO; exceeding 16 per cent of the weight of the coating as awhole.

6. An article of aluminum or an. alloy thereof having formed thereon anintegrally united adsorptive coating having the com position andproperties characteristic of an anodic deposit upon the metal in asulfuric acid electrolyte having a specific gravity between 1.5 and 1.7.

7. An article of aluminum or alloy thereof having an integrally unitedadsorptive SO -containing coating deposited thereon by making thearticle an electrolytic anode in sulfuric acid of about per centstrength, said coating being dyed.

In testimony whereof, I have hereunto affixed my signature.

' HELMER BENGSTON.

